What Size AC Do You Need? Load Calculations Explained

AC capacity is measured in tons — one ton is the cooling power to remove about 12,000 BTU of heat per hour. Homes typically need somewhere between one and a half and five tons, and the difference between the right size and the wrong one shows up every day in comfort and humidity.

This guide explains how the size should be decided, why the old unit is not a reliable guide, and what to ask when you get a recommendation.

Rules like "one ton per 500 square feet" ignore what actually drives the load: how much sun hits the house, the windows, the insulation, the ceiling height, how many people live there and how airtight the home is.

Two houses of the same size on the same street can need different systems if one faces west with large windows and the other sits in the shade.

A load calculation (often called Manual J) works out how much heat enters each room under design conditions. It looks at:

• Floor area and ceiling height of every room
• Wall, ceiling and roof insulation
• Window size, type and which way each one faces
• Shade, overhangs and trees
• Air leakage and duct location
• Occupants and heat from appliances

The result is a cooling load in BTU per hour, which then points to the equipment size.

An oversized system cools the air fast and shuts off before it has run long enough to pull moisture out. The thermostat is satisfied, but the house feels clammy, and the humidity can lead to musty smells.

It also starts and stops more often, which wears parts faster. In a humid climate, a correctly sized system that runs longer, steadier cycles usually feels better than a bigger one.

The old system may have been oversized from the start, or the house may have changed since: new windows, added insulation, a converted garage, a new roof. Matching the old tonnage repeats any old mistake.

A system moves a certain amount of air per ton. If the ducts are too small or leaky for that airflow, a correctly sized system still underperforms and gets noisy. That is why AC installation includes a look at the ductwork, and sometimes duct work before the new unit goes in.

• How was the size worked out — was the house measured?
• What did the calculation say the load is?
• Will the existing ducts carry the airflow this size needs?
• How will this system handle humidity?

Financing is available for new systems, so the decision can be about the right equipment rather than whatever happens to fit the old pad.

Some systems can run at a lower speed most of the time and speed up only on the hottest afternoons. Running slower for longer is exactly what pulls moisture out of Florida air, so this kind of equipment can suit homes where humidity is the main complaint.

It is not the right answer for every home, and it still needs the correct size. A variable-speed system that is oversized behaves much like any other oversized system.

• West-facing glass and sliding doors
• A dark roof or thin attic insulation
• Ducts running through a hot attic
• A porch or lanai converted into living space
• Open living areas with high ceilings

Each of these can move the right size up or down, which is why two similar-looking houses on the same street can end up with different equipment.

• The approximate size of the home and how many floors
• The size of the current system, if you know it
• Rooms that are always too hot or too cold
• Whether the house feels humid even when cool
• Any changes since the old system went in: additions, windows, insulation

A correctly sized system still depends on care to keep its capacity: clean coils, a clean filter and a correct refrigerant charge.

• Keep supply registers open so the design airflow reaches every room
• Change filters on schedule
• Book a tune-up once a year